Re: [RFC PATCH] mm: kasan: suppress soft lockup in slub when !CONFIG_PREEMPT

[Yang Shi]

On 12/8/17 1:16 AM, Andrey Ryabinin wrote:
> On 12/08/2017 11:26 AM, Dmitry Vyukov wrote:
> > On Fri, Dec 8, 2017 at 12:40 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > > On Fri, Dec 08, 2017 at 07:30:07AM +0800, Yang Shi wrote:
> > > > When running stress test with KASAN enabled, the below softlockup may
> > > > happen occasionally:
> > > >
> > > > NMI watchdog: BUG: soft lockup - CPU#7 stuck for 22s!
> > > > hardirqs last  enabled at (0): [<          (null)>]      (null)
> > > > hardirqs last disabled at (0): [] copy_process.part.30+0x5c6/0x1f50
> > > > softirqs last  enabled at (0): [] copy_process.part.30+0x5c6/0x1f50
> > > > softirqs last disabled at (0): [<          (null)>]      (null)
> > >
> > > > Call Trace:
> > > >   [] __slab_free+0x19c/0x270
> > > >   [] ___cache_free+0xa6/0xb0
> > > >   [] qlist_free_all+0x47/0x80
> > > >   [] quarantine_reduce+0x159/0x190
> > > >   [] kasan_kmalloc+0xaf/0xc0
> > > >   [] kasan_slab_alloc+0x12/0x20
> > > >   [] kmem_cache_alloc+0xfa/0x360
> > > >   [] ? getname_flags+0x4f/0x1f0
> > > >   [] getname_flags+0x4f/0x1f0
> > > >   [] getname+0x12/0x20
> > > >   [] do_sys_open+0xf9/0x210
> > > >   [] SyS_open+0x1e/0x20
> > > >   [] entry_SYSCALL_64_fastpath+0x1f/0xc2
> > >
> > > This feels like papering over a problem.  KASAN only calls
> > > quarantine_reduce() when it's allowed to block.  Presumably it has
> > > millions of entries on the free list at this point.  I think the right
> > > thing to do is for qlist_free_all() to call cond_resched() after freeing
> > > every N items.
> >
> >
> > Agree. Adding touch_softlockup_watchdog() to a random low-level
> > function looks like a wrong thing to do.
> > quarantine_reduce() already has this logic. Look at
> > QUARANTINE_BATCHES. It's meant to do exactly this -- limit amount of
> > work in quarantine_reduce() and in quarantine_remove_cache() to
> > reasonably-sized batches. We could simply increase number of batches
> > to make them smaller. But it would be good to understand what exactly
> > happens in this case. Batches should on a par of ~~1MB. Why freeing
> > 1MB worth of objects (smallest of which is 32b) takes 22 seconds?
>
> I think the problem here is that kernel 4.9.44-003.ali3000.alios7.x86_64.debug
> doesn't have 64abdcb24351 ("kasan: eliminate long stalls during quarantine reduction").
>
> We probably should ask that commit to be included in stable, but it would be good to hear
> a confirmation from Yang that it really helps.

Thanks, folks. Yes, my kernel doesn't have this commit. It sounds the 
commit batches the quarantine to smaller group. I will run some tests 
against this commit to see if it could help. Reading the code tells me 
it is likely to help.

Yang

>